Picosecond laser apparatus and methods for treating target tissues with same

ABSTRACT

Apparatuses and methods are disclosed for applying laser energy having desired pulse characteristics, including a sufficiently short duration and/or a sufficiently high energy for the photomechanical treatment of skin pigmentations and pigmented lesions, both naturally-occurring (e.g., birthmarks), as well as artificial (e.g., tattoos). The laser energy may be generated with an apparatus having a resonator with the capability of switching between a modelocked pulse operating mode and an amplification operating mode. The operating modes are carried out through the application of a time-dependent bias voltage, having waveforms as described herein, to an electro-optical device positioned along the optical axis of the resonator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase under 35 U.S.C. §371 of International Application No. PCT/US2013/032228, filed Mar. 15, 2013, which claims priority to and the benefit of U.S. Provisional Patent Application No. 61/625,961 entitled “Picosecond Laser Apparatus and Methods for Treating Dermal Tissues With Same”, filed Apr. 18, 2012, the entire disclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an apparatus and methods for delivering laser energy having a short pulse duration (e.g., less than about 1 nanosecond) and high energy output per pulse (e.g., greater than about 200 millijoules).

SUMMARY OF THE INVENTION

Disclosed herein are subnanosecond pulse duration laser systems, which are useful for a variety of cosmetic and medical treatments. A subnanosecond pulse duration laser apparatus includes: (a) a resonator having a first mirror at one end of said resonator and a second mirror at the opposite end of said resonator, wherein both said first mirror and said second mirror are substantially totally reflective; and (b) a lasing medium, an ultrafast switching element and a polarizing element along the optical axis of said resonator, wherein a first drive circuit is connected to a first end of the ultrafast switching element and a second drive circuit is connected to a second end of the ultrafast switching element, wherein said apparatus generates a modelocked pulse by the first circuit applying a periodic voltage waveform to the first end of the ultrafast switching element and then amplifies the modelocked pulse by the first circuit applying a first constant voltage to the first end of the ultrafast switching element and maintaining an effective reflectivity of the second mirror at substantially 100%, and then extracts the amplified modelocked pulse by the second circuit applying a second constant voltage to the second end of the ultrafast switching element and maintaining an effective reflectivity of the second mirror at substantially 0%.

Subnanosecond pulse duration laser systems that are useful for cosmetic and medical applications provide pulsed laser energy that delivers at least about 100 mJ/pulse and up to about 800 mJ/pulse. An exemplary output energy has about 200 mJ/pulse. Likewise, such subnanosecond laser systems have pulse durations of about 100 picoseconds to less than 1000 ps, and preferably pulse durations of about 200 ps to 600 ps, or about 400-500 ps.

Subnanosecond laser systems are particularly useful in the treatment of skin and skin lesions. For example, tattoo removal requires delivery of subnanosecond laser pulses to the dermis, where photomechanical damage to ink particles facilitates the removal of the tattoo by the subject's immune system. Colored tattoos or heavily shaded tattoos are easily treated by such subnanosecond systems, using fewer treatments to achieve a desired reduction in the visible appearance of the tattoo. Other skin treatments include benign pigmented lesions, where applying subnanosecond pulsed laser energy to benign pigmented lesions decreases the visible appearance of the pigmented lesions. Similar effects are seen in the treatment of vascular lesions, where applying subnanosecond pulsed laser energy to the vascular lesion thereby decreases the visible appearance of the vascular lesions.

In addition to pigmented lesions, scars, wrinkles and striae are also treatable using subnanosecond pulsed laser energy. The laser energy can be used to debulk the scars, and it creates generally areas in the tissue of microdamage from photomechanical effects. This has tissue-inductive effects, resulting in the evening-out of tissue surfaces and the improvement in coloration and texture of the target tissue. In certain aspects of the invention, the subnanosecond pulsed laser energy is modified with a lens, thereby producing a treatment beam having a nonuniform energy cross section characterized by of a plurality of regions of relatively high energy per unit area dispersed within a background region of relatively low energy per unit area. Such systems outputting subnanosecond pulse laser energy in a nonuniform beam deliver sufficient energy to target tissue illuminated by regions of relatively high energy per unit area to heat the so-illuminated portions of the target tissue to a first temperature T1 and wherein the substantially uniform background region of relatively low energy per unit area delivers sufficient energy to target tissue illuminated by the regions of relatively high energy per unit area to heat the so illuminated portions of the target tissue to a second temperature T2, wherein T2 is less than T1.

In other embodiments, the invention provides for wavelength shifted subnanosecond pulse laser, that can be frequency matched to the absorption spectrum of a skin pigment or tattoo ink. A method for shifting the wavelength of a subnanosecond pulse laser apparatus includes maintaining a relatively constant pulse duration by using the pulse as a pump for a laser resonator with a short roundtrip time by including a laser crystal with high absorption coefficient at the wavelength of the short pulse; where the round trip time of the short laser resonator is substantially shorter than the pumping laser pulse duration.

The invention will be more completely understood through the following detailed description, which should be read in conjunction with the attached drawings. Detailed embodiments of the invention are disclosed herein, however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the invention in virtually any appropriately detailed embodiment.

DETAILED DESCRIPTION

Lasers are recognized as controllable sources of radiation that is relatively monochromatic and coherent (i.e., has little divergence). Laser energy is applied in an ever-increasing number of areas in diverse fields such as telecommunications, data storage and retrieval, entertainment, research, and many others. In the area of medicine, lasers have proven useful in surgical and cosmetic procedures where a precise beam of high energy radiation causes localized heating and ultimately the destruction of unwanted tissues. Such tissues include, for example, subretinal scar tissue that forms in age-related macular degeneration (AMD) or the constituents of ectatic blood vessels that constitute vascular lesions.

Most of today's aesthetic lasers rely on heat to target tissue and desired results must be balanced against the effects of sustained, elevated temperatures. The principle of selective photothermolysis underlies many conventional medical laser therapies to treat diverse dermatological problems such as leg veins, portwine stain birthmarks, and other ectatic vascular and pigmented lesions. The dermal and epidermal layers containing the targeted structures are exposed to laser energy having a wavelength that is preferentially or selectively absorbed in these structures. This leads to localized heating to a temperature (e.g., to about 70 degrees C.) that denatures constituent proteins or disperses pigment particles. The fluence, or energy per unit area, used to accomplish this denaturation or dispersion is generally based on the amount required to achieve the desired targeted tissue temperature, before a significant portion of the absorbed laser energy is lost to diffusion. The fluence must, however, be limited to avoid denaturing tissues surrounding the targeted area.

Fluence, however, is not the only consideration governing the suitability of laser energy for particular applications. The pulse duration and pulse intensity, for example, can impact the degree to which laser energy diffuses into surrounding tissues during the pulse and/or causes undesired, localized vaporization. In terms of the pulse duration of the laser energy used, conventional approaches have focused on maintaining this value below the thermal relaxation time of the targeted structures, in order to achieve optimum heating. For the small vessels contained in portwine stain birthmarks, for example, thermal relaxation times and hence the corresponding pulse durations of the treating radiation are often on the order of hundreds of microseconds to several milliseconds.

Cynosure's PicoSure™ brand laser system is the first aesthetic laser to utilize picosecond technology which delivers laser energy at speeds measured in trillionth of seconds (10⁻¹²). PicoSure systems deliver both heat and mechanical stress to shatter the target from within before any substantial thermal energy can disperse to surrounding tissue. Clinical results show a higher percentage of clearance achieved in fewer treatments. PicoSure systems, employing Pressure Wave™ technology, is useful for multiple aesthetic indications such as pigmented lesions and multi-colored tattoo removal as well as dermal rejuvenation.

An exemplary PicoSure™ brand picosecond laser apparatus is detailed in our U.S. Pat. Nos. 7,586,957 and 7,929,579, incorporated herein by reference. Such a laser apparatus provides for extremely short pulse durations, resulting in a different approach to treating dermal conditions than traditional photothermal treatments. Laser pulses having durations below the acoustic transit time of a sound wave through targeted particles are capable of generating photomechanical effects through pressure built up in the target particles. Photomechanical processes can provide commercially significant opportunities, particularly in the area of treating skin pigmentations. Coupled with high energy output, such lasers described above are particularly suitable for the following exemplary applications. Table 1. provides fluence values for particular spot sizes, for a subnanosecond pulse laser outputting about 200 mJ/pulse.

TABLE 1 PicoSure Fluence Spot size J/cm{circumflex over ( )}2 2 6.369 2.5 4.076 3 2.831 3.5 2.080 4 1.592 4.5 1.258 5 1.019 5.5 0.842 6 0.708 8 0.398 10 0.255 A. Tattoo Removal

The incidence of tattoos in the U.S. and other populations, for example, continues at a significant pace. Because tattoo pigment particles of about 1 micron in diameter or less may be cleared from the body via ordinary immune system processes, stable tattoos are likely composed of pigment particles having diameters on the order of 1-10 microns or more. The acoustic transit time of a sound wave in a particle of tattoo pigment is calculated by dividing the radius of the particle by the speed of sound in the particle. As the speed of sound in tattoo pigment, as well as many solid media is approximately 3000 meters/second, the acoustic transit time across such particles, and consequently the laser pulse duration required to achieve their photomechanical destruction of the tattoo pigment is as low as hundreds of picoseconds.

In addition to such short pulse durations, high energy laser pulses are needed for significant disruption of tattoo pigment particles. Given that most tattoos are on the order of multiple centermeters in size, the ideal laser for tattoo removal should ideally employ a beam having a relatively large spot size. Fluences of several joules per square centimeter and treatment spot sizes of a few millimeters in diameter translate to a desired laser output with several hundred millijoules (mJ) per pulse or more, and are suitable for tattoo removal.

An exemplary sub-nanosecond tattoo removal laser apparatus as described in our U.S. Pat. Nos. 7,586,957 and 7,929,579 and as described herein is used to generate pulsed laser energy having a subnanosecond pulse duration of about 100-950 picoseconds, preferably with an energy delivery of about 200-750 mJ/pulse. Such exemplary subnanosecond laser apparatus includes a resonator with two substantially totally reflective mirrors at opposite ends of its optical axis. An alexandrite crystal lasing medium, a polarizer, and a Pockels cell are positioned along this optical axis. An optical flashlamp is also included for pumping the alexandrite lasing medium, which generates laser energy having a wavelength in the range of about 700-950 nm.

The pulsed laser energy described above is generated by pumping the lasing medium and first establishing a modelocked pulse oscillating in the resonator. In the modelocked pulse operating mode, a time-dependent voltage waveform, as described herein, is applied to the Pockels cell. This waveform results from the sum of a constant baseline voltage and a time-dependent differential voltage. The baseline voltage is in the range of 1000-1500 volts (representing 40%-60% of the Pockels cell quarter wave voltage, or 2500 volts) and is negatively offset or modulated by the time-dependent differential voltage, having an amplitude in the range of 250-750 volts (representing 10%-30% of the Pockels cell quarter wave voltage). The period of the resulting voltage waveform is in the range from 5-10 ns and is equal to the round trip time of the oscillating laser energy in the resonator. The voltage applied to the Pockels cell is thus modulated at a frequency in the range from 100-200 MHz.

Subsequently, the modelocked pulse established as described above is amplified by discharging the Pockels cell to essentially 0 volts. Oscillating laser energy is reflected between the mirrors at each end of the resonator, with essentially no losses. This laser energy therefore rapidly increases in amplitude by extracting energy previously pumped and stored in the alexandrite crystal during modelocking. When the laser energy has reached the desired energy level as indicated above, it is extracted from the resonator by applying the quarter wave voltage of 2500 volts to the Pockels cell.

The switching electronics used to operate the laser in modelocked pulse and amplification modes, and finally to extract the amplified pulse as discussed above, comprise five MOFSET switches, two high speed diodes, and three voltage sources having voltages V1 in the range of +1000 to +1500 volts, V2 in the range of +250 to +750 volts, and V3 in the range of −1000 to −1500 volts. The switches, diodes, and voltage sources are configured as shown above.

Laser energy having the pulse duration and energy as described is applied to a patient undergoing treatment for the removal of a tattoo. This laser energy is applied over the course of a 30-minute treatment session to all areas of the skin having undesired tattoo pigment particles. Photomechanical disruption of these particles is effected using the short pulse duration (below the transit time of a sound wave through the targeted tattoo pigment particles), together with a fluence in the range of 2-4 J/cm². This fluence is achieved in the above device with a laser energy spot diameter of about 5 mm.

Most if not all of the undesired tattoo pigment particles are effectively photomechanically disrupted, destabilized, and/or broken apart using one or two treatments. As a result, the disrupted particles are cleared from the body via normal physiological processes, such as the immune response. The tattoo is thus eventually cleared from the skin with no remaining visible signs. Such subnanosecond laser devices are particularly well suited for removal of tattoos having colored inks, colors being far more recalcitrant to treatment with traditional laser treatments than black inks. Likewise, tattoos having heavy coloration or shading are more amenable to treatment using subnanosecond laser systems. Similarly, subnanosecond lasers provide for tattoo removal with fewer treatments than compared to traditional laser systems.

B. Benign Pigmented Lesions

Numerous types of benign pigmented lesions of the skin, connective tissue, mucosal tissue and vasculature are treatable using the subnanosecond laser systems described herein.

Nevi are a broad category of generally well circumscribed and chronic lesions of the skin that can be congenital or develop later in life. Vascular nevi such as hemangioma, are derived from structures of the blood vessels. Epidermal nevi such as seborrheic keratoses are derived from keratinocytes. Connective tissue nevi are derived from connective tissues. Melanocytic nevi such as nevomelanocytic nevi are pigmented lesions that morphologically can be flat macules or raised papules, and are characterized by clusters of melanocytes. In addition to the above, dermal melanocytoma (blue nevi), acral nevi, nevus spilus (also known as speckled lentiginous nevus), nevus of Ota/Ito and Becker's nevus are all exemplary and non-limiting nevi that are suitable for treatment using the above laser apparatus.

Lentigines are similarly treatable. These are pigmented spots on the skin typically small and with a clearly-defined edge, surrounded by normal-appearing skin. These are benign melanocytic hyperplasias that are linear rather than raised, with the melanocytes generally restricted to the cell layer directly above the basement membrane of the epidermis. Lentigines also appear in mucosal tissues. Lentigines are distinguished from ephelids (freckles) based on melanocyte proliferation, where lentigines display an increased number of melanocytes but ephelids have normal numbers of melanocytes that overexpress melanin.

Other forms of congenital dermal pigmentation are equally suitable to treatment using the above laser apparatus, usually being larger areas requiring larger laser spot sizes. For example, café au lait macules, congenital dermal melanocytosis, and dermal melanocytosis are all exemplary types of benign, flat, pigmented birthmarks, generally with wavy borders and irregular shapes. Other exemplary pigmented lesions develop as one ages, or due to hormonal changes, infection or treatment with pharmaceutical agents. For example, melasma a/k/a chloasma faciei (colloquially “the mask of pregnancy”) is a tan or dark skin discoloration. Postinflammatory hyperpigmentation (also known as postinflammatory hypermelanosis) can result from natural or iatrogenic inflammatory conditions, and are commonly caused by increased epidermal pigmentation. This can occur through increased melanocyte activity or by dermal melanosis from melanocyte damage with melanin migration from the epidermis into the dermis. Drug-induced pigmentation of the skin may occur as a consequence of drug administration, related to deposition of the drug in the tissues. Minocycline is known for this effect. Pigmented lesions aren't confined to the dermal tissues. Ochronosis is a pigmented lesion caused by the accumulation of homogentisic acid in connective tissues. In addition, melanonychia is aberrant pigmentation of the normal nail plate. These exemplary conditions are all suitable to treatments using the above described laser systems.

To treat the above conditions, it is generally accepted that destruction of melanosomes is pulse-width-dependent. Using traditional laser systems, pulse durations of between 40 nanoseconds and 750 nanoseconds have been shown to be effective, but longer pulse durations (eg, 400 microseconds) do not appreciably damage the melanosomes. Likewise, Q-switched Nd:YAG laser systems have shown immediate skin whitening with threshold energy exposures generating fluence values of 0.11, 0.2, and 1 J/cm² respectively at 355, 532, and 1064 nm wavelengths.

Melanin has a broad absorbtion spectrum, and lasers emitting at wavelengths of about 500-1100 nm provide for good skin penetration and selective melanosome absorption without undue hemoglobin absorption. Exemplary lasers include 510-nm pulsed dye, 532-nm frequency-doubled Nd:YAG, 694-nm ruby, 755-nm alexandrite, and near-infrared Nd:YAG lasers emitting at 1064 nm. Other lasers have been used successfully to treat pigmented lesions, including argon, krypton, copper, carbon dioxide, and Er:YAG lasers, but with these systems there is a trade-off between pigment destruction and collateral damage to other chromophores and tissues.

An exemplary subnanosecond laser system for treating pigmented lesions is described by the above apparatus generating pulsed laser energy having a pulse duration of about 100-950 picoseconds with an energy output of about 200-750 mJ/pulse. Laser energy having a wavelength in the range of 700-950 nm provides excellent specificity for melanin. Photomechanical disruption is effected using the short pulse duration (below the transit time of a sound wave through the targeted pigment particles), together with a fluence in the range of 2-4 J/cm². This fluence is achieved with a laser energy spot diameter of about 5 mm, which can be changed according to the area of the target. Treatment times will vary with the degree of pigmentation and the shapes of the targets. In addition to decreasing the visible appearance of pigmented lesions, photomechanical microdamage to target tissues caused by subnanosecond pulses promotes a healing response that can decrease the size and shape of the lesion.

C. Vascular Lesions

Vascular lesions refer to a broad category of pigmented malformations, generally congenital, that are due to localized defects of vascular morphogenesis. These include capillary, venous, arteriovenous, and lymphatic malformations as well as lesions involving only the skin and subcutaneous tissues. Exemplary non-limiting examples include capillary vascular malformation, telangiectasis, cherry angioma, angiofibroma, dyschromia, port wine stain birthmarks, strawberry hemangiomas, rosacea, pyogenic granuloma and other vascular malformations.

The targeted chromophore for vascular lesions is intravascular oxyhemoglobin, with maximal light absorption occurring in the range of yellow and green light, i.e., at 418, 542, and 577 nm and in the near-infrared spectrum. Traditional approaches to treating vascular lesions involved pulsed dye lasers, typically operating at wavelengths such as 585-nm or 595-nm, frequency-doubled Nd:YAG lasers at wavelengths of 532 nm, and infrared lasers such as alexandrite or diode lasers having wavelengths of 1064 nm. Histologically, the targets of treatment are postcapillary venules, capillaries, or arterioles, generally at depths range from 200 to 300 μm. Accordingly, a deeper penetration of the laser beam is desirable, particularly where heating of the surface skin can lead to excessive scarring. Typical treatment parameters involve fluences of 8 to 10 J/cm², and a 5-10 mm spot size.

An exemplary system to treat vascular lesions is described by an apparatus generating pulsed laser energy having a pulse duration of about 100-950 ps with energies of about 200-750 mJ/pulse. Laser energy having a wavelength in the range of 500-600 nm provides excellent specificity for oxyhemoglobin. Photomechanical disruption is effected using the short pulse duration (below the transit time of a sound wave through the targeted pigment particles), together with a fluence in the range of 7-10 j/cm². This fluence is achieved with a laser energy spot diameter of about 5 mm, which can be changed according to the area of the target. Treatment times will vary with the degree of pigmentation and the shapes of the targets.

D. Scar Tissue

Various types of scarring are treatable using lasers. Exemplary non-limiting types include hypertrophic scars, keloids and atrophic scars. Hypertrophic scars are cutaneous deposits of excessive amounts of collagen. These give rise to a raised scar, and are commonly seen at prior injury sites particularly where the trauma involves deep layers of the dermis, i.e., cuts and burns, body piercings, or from pimples. Hypertrophic scars commonly contain nerve endings are vascularized, and usually do not extend far beyond the boundary of the original injury site.

Similarly, a keloid is a type of scar resulting from injury, that is composed mainly of either type III or type I collagen. Keloids result from an overgrowth of collagen at the site of an injury (type III), which is eventually replaced with type 1 collagen, resulting in raised, puffy appearing firm, rubbery lesions or shiny, fibrous nodules, which can affect movement of the skin. Coloration can vary from pink to darker brown.

Atrophic scarring generally refers to depressions in the tissue, such as those seen resulting from Acne vulgaris infection. These “ice pick” scars can also be caused by atrophia maculosa varioliformis cutis (AMVC), which is a rare condition involving spontaneous depressed scarring, on the cheeks, temple area and forehead.

Laser treatments are suitable for hypertrophic and atrophic scars, and keloids, and common approaches have employed pulsed dye lasers in such treatments. In raised scars, this type of therapy appears to decrease scar tissue volume through suppression of fibroblast proliferation and collagen expression, as well as induction of apoptotic mechanisms. Combination treatment with corticosteroids and cytotoxic agents such as fluorouracil can also improve outcome. In atrophic scars, treatments can even out tissue depths.

Striae (stretch marks) are a form of scarring caused by tearing of the dermis. They result from excess levels of glucocorticoid hormones, which prevent dermal fibroblasts from expressing collagen and elastin. This leads to dermal and epidermal tearing. Generally, 585-nm pulsed dye laser treatments show subjective improvement, but can increase pigmentation in darker skinned individuals with repeated treatments. Fractional laser resurfacing using scattered pulses of light has been attempted. This targets small regions of the scar at one time, requiring several treatments. The mechanism is believed to be the creation of microscopic trauma to the scar, which results in new collagen formation and epithelial regeneration. Similar results can be achieved, albeit to the total scar, through the use of modified laser beams as described in our U.S. Pat. No. 7,856,985, detailing the use of non-uniform beam radiation to create within the beam area, discrete microtrauma sites against a background of tissue inducing laser radiation.

An exemplary system for treating scars is described by the above apparatus generating pulsed laser energy having a pulse duration of about 100-950 ps with energies of about 200-750 mJ/pulse. Laser energy having a wavelength in the range of 500-1100 nm provides excellent specificity for collagen. Photomechanical disruption of the scar tissue is effected using the short pulse duration (below the transit time of a sound wave through the targeted tissue), together with a fluence in the range of 2-4 J/cm². This fluence is achieved with a laser energy spot diameter of about 5 mm, which can be changed according to the area of the target. Treatment times will vary with the degree of scarring and the shapes of the targets. The photomicrodamage from subnanosecond pulses can debulk the scar, resolve coloration differences between it and healthy tissues, and promotes tissue healing responses which have the effect of softening the scar. Modifying the output beam as described in U.S. Pat. No. 7,856,985 provides a particularly useful approach to reducing scar appearance and inducing epithelial restoration within the scar.

E. Dermal Rejuvenation

A non-uniform output beam is delivered to tissue from a source of light as described in our patent applications U.S. Ser. Nos. 11/347,672; 12/635,295; 12/947,310, and PCT/US10/026432. The non-uniform beam is characterized by a cross-section corresponding to an array of relatively small, relatively high-intensity, spaced-apart central regions superimposed on a relatively large, and relatively low-intensity background region. Operatively, this produces within the area of the beam, relatively hotter regions and relatively cooler regions. This non-uniform beam provides for unique physiological effects as compared to standard uniform output laser beams that demonstrate relative uniform energy output across the planar surface of the beam. Such effects are related to the fluence and duration of the light pulse, and include various quantifiable physiological effects. Exemplary temperature dependent effects include but are not limited to parakeratosis, perivascular mononuclear infiltration, keratinocyte necrosis, collagen denaturation, and procollagen expression in dermal cells. Other cellular markers (e.g., nucleic acids and proteins) are useful in detecting more subtle responses of skin to less aggressive treatments.

Various combinations of wavelength, power, spot size, treatment duration and recovery intervals are possible, and the particular combination is selected based on the desired therapeutic effect. For example, in treating age spots and pigmentation a device wavelength is chosen to be preferentially absorbed by melanin (between 400 nm and 1400 nm, and more preferably 500 nm to 1100 nm). Accordingly, an exemplary device for such purposes has a wavelength of about 750 nm, a pulse duration of about 500 to 900 ps and an overall treatment area of 1 cm² that is output as a non-uniform beam characterized by a cross-section corresponding to an array of relatively small, relatively high intensity, spaced-apart central regions superimposed on a relatively large, relatively low intensity background region. If such exemplary device has about 0.2 J of energy delivered into the treatment area, or about 0.2 J/cm² average fluence. Using a lens that renders the output beam non-uniform, delivering relatively high intensity spaced-apart central regions at 1 mm center-to-center distances surrounded by low intensity background regions, in such device, there are about 115 discrete subzones (e.g., combined areas of relatively high and relatively low intensity) per square centimeter in that arrangement, which results in about 1.73 mJ delivered to each subzone. Within each subzone, if the high intensity spaced-apart central region is about 120 μm in diameter and approximately 80% of the energy is delivered into the high intensity spaced-apart central regions, then the fluence within each high intensity region is approximately 12.2 J/cm². That fluence value in the device is comparable to the treatment fluence delivered by high-powered Alexandrite uniform spot lasers having pulse durations of about 50 ns, which are the systems commonly used to treat uneven skin pigmentation in clinical settings by medically trained professionals. Unlike uniform-beam devices, using the non-uniform beam technology for any individual treatment session, only a relatively small percentage of the irradiated skin surface is actually treated with high intensity light, and thereby only a subpopulation of melanocytes receive a cellular disruptive dose of thermal energy, leading to a relatively smaller percentage of melanocyte damage per treatment area compared to uniform beam treatments. This advantageously reduces any sharp boundaries between treated and untreated skin, thereby reducing the need for special operator skills and techniques.

Good cosmetic effects can be produced by such non-uniform irradiation of tissues, due to differential effects occurring in both the relatively high intensity spaced-apart central regions of the beam and in the relatively low intensity background region. By way of illustration, within the spaced-apart central regions it is possible to cause relatively localized heating of tissues therein to a temperature T1 sufficient to heat up the melanocytes to a temperature sufficient to disrupt cellular processes (e.g., about 45 degrees C. or higher), impair their function and decrease their pigment output. Simultaneously during the treatment, within the low intensity background regions at a lower relative temperature T2 (e.g., less than 45 degrees C. to about 35 degrees C.), cellular growth and collagen production is induced without causing undesirable thermal effects to the treated tissue within the lower energy regions. The result of such treatment is an improvement to both skin texture and coloration. Other differential effects on tissues can be realized as well. By way of further example, temperatures at about 70 degrees C. can serve to denature collagen, so within the spaced-apart central regions it is possible to cause relatively localized heating of tissues therein to a temperature T1 sufficient to remodel collagen structures, while simultaneously within the low intensity background regions at a lower relative temperature T2, collagen production is induced without causing undesirable thermal effects to the treated tissue within the background regions.

Likewise, by decreasing the amount of energy delivered by the beam it is possible to select for specific thermal effects on tissues. For example, in our U.S. Pat. No. 7,856,985 we disclose collagen remodeling at temperatures where T1 is approximately 70 degrees C. or greater while the irradiated tissues in the cooler regions of the beam (e.g., at temperature T2) are not substantially adversely affected. The device used generating a non-uniform beam output, permits more selective application with less collateral tissue damage. However, for reducing age spots and evening skin pigmentation, melanocyte cell membrane damage with consequent cellular disruption is achieved at lower T1 temperatures of approximately 45-50 degrees C. (unless such heating is quite transient). Higher temperatures are suitable, and cause permanent disruption of melanocytes, but above 50 degrees C. more extensive thermal effects are seen in the tissue, that must be evaluated against therapeutic benefits. Below the temperature threshold for causing cellular damage and disruption, positive effects on skin tone are seen. At a T1 temperature of less than about 50 degrees C., cells are not substantially damaged but are still induced to generate a healing response, and express elastin, procollagen, keratin and other markers for dermal rejuvenation. So a device generating regions capable of elevating tissue temperatures to a T1 of about 45 degrees C. against a background T2 of about 37 degrees.

The overall effect of treatments on skin tone, wrinkling and pigmentation provide the best indication of therapeutic efficacy, but such treatments also leave histological evidence that can be discerned. At higher energies, thermal damage is easy to detect. For more moderate energies, microthermal damage can produce effects that are seen with magnification although erythema provides a good marker for microthermal injury and it does not require microscopic examination of tissues from the treatment site. Generally, in the absence of any visually observable erythema, the cellular effects will be more subtle, or may take longer to manifest themselves or may require multiple treatments before visual improvement of the skin is seen. At lower output energies, shorter pulse durations, and longer intervals between treatments, it is advantageous to use more sensitive techniques to assay for cellular changes. Certain techniques provide for quantitative analysis, which are correlated to describe a dose-response relationship for the non-uniform beam, as it is used in dermal rejuvenation applications. Such techniques include but are not limited to RT-PCR and/or real-time PCR, either of which permits quantitative measurements of gene transcription, useful to determine how expression of a particular marker gene in the treated tissues changes over time. In addition to nucleic acid-based techniques, quantitative proteomics can determine the relative protein abundance between samples. Such techniques include 2-D electrophoresis, and mass spectroscopy (MS) such as MALDI-MS/MS and ESI-MS/MS. Current MS methods include but are not limited to: isotope-coded affinity tags (ICAT); isobaric labeling; tandem mass tags (TMT); isobaric tags for relative and absolute quantitation (iTRAQ); and metal-coded tags (MeCATs). MeCAT can be used in combination with element mass spectrometry ICP-MS allowing first-time absolute quantification of the metal bound by MeCAT reagent to a protein or biomolecule, enabling detection of the absolute amount of protein down to attomolar range.

An exemplary system for dermal rejuvenation is described by the above apparatus generating pulsed laser energy having a pulse duration of about 100-950 ps with energies about 200-750 mJ/pulse. Laser energy having a wavelength in the range of 500-1100 nm provides excellent specificity for collagen. Photomechanical disruption of the target tissue is effected using the short pulse duration (below the transit time of a sound wave through the targeted tissue), together with a fluence in the range of 2-4 J/cm². This fluence is achieved with a laser energy spot diameter of about 5 mm, which can be changed according to the area of the target. Treatment times will vary according to the desired effects. Modifying the output beam as described in U.S. Pat. No. 7,856,985 provides a particularly useful approach to rejuvenating tissue and inducing collagen and epithelial cell restoration within the tissue.

F. Frequency Shifting

The subnanosecond laser systems may include a frequency shifting apparatus, which can be matched to the absorbtion spectrum of endogenous skin pigmentation or exogenous tattoo pigments to be targeted. Such a system comprises a rare earth doped laser gain crystal for example Nd:YVO4, and a frequency doubling crystal for example KTP. Rare earth doped laser crystals that generate a polarized laser beam like Nd:YVO4 are preferred. Crystals like Nd:YAG or Nd doped glasses can be used with an additional polarizing element in the resonator. The input side of the Nd:YVO4 crystal is AR coated for the alexandrite wavelength and HR coated for 1064 nm. The output side of the crystal is HR coated for the alexandrite wavelength and has approximately 20 to 70% reflectivity at 1064 nm. The Nd:YVO4 crystal length is chosen so that it absorbs most (greater than 90%) of the alexandrite laser pulse in the two passes through the crystal. For Nd doping in the range 1 to 3% the Nd:YVO4 crystal can be chosen to be around 3 mm long. There are no other optical elements in the resonator and the resonator length is equal to the crystal length 3 mm. That means the resonator round-trip time is around 36 ps—this is substantially less than the pulse duration of the alexandrite pumping pulse (around 500 to 800 ps). The 1064 nm pulse generated in the very short round trip time Nd:YVO4 resonator will be slightly longer than the pumping alexandrite pulse and it will be shorter than 1000 ps. The quantum defect will account for a 30% pulse energy loss and another 15% of the energy is likely to be lost due to coatings, crystal and geometry imperfection for an overall energy conversion efficiency of around 50 to 60%. That means a 100 mJ pulse energy can be expected at 1064 nm. Conservatively it is estimated that the second harmonic conversion in the KTP crystal will be around 50% and a 50 mJ pulse energy can be expected at 532 nm. The red tattoo pigments have high absorption at 532 nm. A 50 mJ 532 nm pulse with a pulse duration less than 1000 ps is expected to be effective at disrupting red tattoo granules.

When the Nd:YAG crystal or Nd doped glasses are used in the short resonator an extra polarizing element has to be used to generate a polarized 1064 nm pulse. The cross-section depicts a short Nd:YAG resonator consisting of two identically shaped crystals with one face cut at an angle that is AR coated for the alexandrite wavelength and polarized coating for 1064 nm (high p transmission). The flat faces of the two Nd:YAG crystals have different coatings—one is AR coated at 755 nm and HR coated at 1064 nm and the other is HR coated for 755 nm and has an output coupler reflectivity around 50 to 80% for 1064 nm. The higher output coupler reflectivity for the Nd:YAG crystal compared to the Nd:YVO4 crystal is due to the lower gain cross-section in Nd:YAG.

Generating a short 1064 nm pulse depends on two factors. One is the pulse duration of the pumping alexandrite pulse. Shorter pumping pulses will lead to shorter generated pulses at 1064 nm. The second factor is the 1064 nm resonator round trip time that is determined by the length of the Nd doped crystal. Shorter crystals lead to shorter roundtrip time, however the crystal has to be sufficiently long to absorb greater than 90% of the alexandrite energy. For example, an 8 mm long Nd:YAG resonator would have a 97 ps round trip time. That round trip time is longer than the round trip time that can be achieved with a Nd:YVO4 resonator, but still it is much shorter than the pumping Alexandrite laser pulse duration. One possible way to shorten the crystal length is to tune the alexandrite laser in the range 750 to 760 nm for maximum absorption in the Nd doped crystal and use the minimum possible crystal length. In addition, tuning the alexandrite laser in the range 750 to 757 nm allows for the alexandrite wavelength to be set to avoid the excited state absorption bands in the Nd ion as described by Kliewer and Powell, IEEE Journal of Quantum Electronics vol. 25, page 1850-1854, 1989. 

The invention claimed is:
 1. A method for shifting a wavelength of a subnanosecond pulse laser apparatus comprising: providing a subnanosecond pulse as a pump for a laser resonator with a short roundtrip time, the laser resonator comprising an optical element, the laser resonator having a laser resonator length, wherein the optical element is a laser crystal with high absorption coefficient at the wavelength of the subnanosecond pulse, the laser crystal having a crystal length, wherein the crystal length is equal to the laser resonator length, wherein the roundtrip time of the laser resonator is substantially shorter than a pulse duration of the subnanosecond pulse that was used to pump the laser resonator; and generating, in the laser resonator, a subnanosecond wavelength shifted pulse having at least about 100 mj/pulse energy with a pulse duration such that a shorter subnanosecond pumping pulse used to pump the laser resonator results in a shorter generated subnanosecond pulse.
 2. The method of claim 1, wherein the wavelength shifted pulse is shifted to match an absorption spectrum of a pigment.
 3. The method of claim 2 wherein the pigment is at least one of endogenous skin pigment and exogenous tattoo pigment.
 4. The method of claim 1 wherein the roundtrip time of the laser resonator is at least 20 times shorter than the wavelength shifted pulse duration.
 5. The method of claim 1 wherein the roundtrip time of the laser resonator is at least 13 times shorter than the wavelength shifted pulse duration.
 6. The method of claim 1 wherein the roundtrip time of the laser resonator is at least 5 times shorter than the wavelength shifted pulse duration.
 7. The method of claim 1 wherein the laser crystal is a rare earth doped laser crystal.
 8. The method of claim 1 wherein the laser crystal is one of Nd:YAG and Nd:YVO4.
 9. An apparatus for shifting a wavelength of a subnanosecond pulse laser comprising a laser resonator having a laser resonator length, wherein roundtrip time of light in laser resonator is substantially shorter than a pulse duration of a subnanosecond pulse that is used to pump the laser resonator; and a laser crystal with a high absorption coefficient at a wavelength of the subnanosecond pulse, wherein laser crystal has a crystal length, wherein the crystal length and the laser resonator length are equal, the laser crystal disposed in the laser resonator, the laser resonator being capable of generating a subnanosecond wavelength shifted pulse having at least about 100 mj/pulse energy with a pulse duration such that a shorter subnanosecond pumping pulse used to pump the laser resonator results in a shorter generated subnanosecond pulse.
 10. The apparatus of claim 9 wherein the roundtrip time of the laser resonator is at least 20 times shorter than the subnanosecond pulse duration.
 11. The apparatus of claim 9 wherein the roundtrip time of the laser resonator is at least 13 times shorter than the subnanosecond pulse duration.
 12. The apparatus of claim 9 wherein the roundtrip time of the laser resonator is at least 5 times shorter than the subnanosecond pulse duration.
 13. The apparatus of claim 9 wherein the laser crystal is a rare earth doped laser crystal.
 14. The apparatus of claim 9 wherein the laser crystal is one of Nd:YAG and Nd:YVO4.
 15. The apparatus of claim 9 comprising a frequency doubling crystal that generates a second harmonic conversion of the wavelength generated in the laser resonator.
 16. The apparatus of claim 15 wherein the second harmonic conversion of the wavelength of the subnanosecond pulse laser matches the absorption spectrum of a pigment.
 17. The apparatus of claim 15 wherein the frequency doubling crystal is KTP crystal.
 18. The apparatus of claim 9 wherein an additional polarizing element is used with the laser crystal or the doped glass in the laser resonator.
 19. The apparatus of claim 9 wherein a second harmonic conversion of the wavelength shifted pulse has a 50 mj/pulse energy.
 20. The method of claim 1 wherein a second harmonic conversion of the wavelength shifted pulse has a 50 mj/pulse energy.
 21. The method of claim 1, wherein the laser resonator has a laser crystal with high absorption coefficient at the wavelength of the subnanosecond pulse.
 22. The method of claim 1, wherein the laser resonator has a doped glass with high absorption coefficient at the wavelength of the subnanosecond pulse.
 23. The device of claim 9, wherein the laser resonator has a laser crystal with high absorption coefficient at the wavelength of the subnanosecond pulse.
 24. The device of claim 9, wherein the laser resonator has a doped glass with high absorption coefficient at the wavelength of the subnanosecond pulse. 